Helping hands

Flexible lightweight artificial hands from nickel-titanium alloy

December 08, 2015

The hand is the perfect tool. Evolving over millions of years, its ‘design’ can certainly be said to be mature. It is extraordinarily mobile and adaptable, and the remarkable interaction between the muscles, ligaments, tendons, bones and nerves has long driven a desire to create a flexible device based upon it. Now the properties of nickel are advancing this tool for human and industrial applications.

A research team, led by Professor Stefan Seelecke from Saarland University and the Center for Mechatronics and Automation Technology (ZeMA), both in Saarbrücken Germany, is using a new technology based on the shape-memory properties of a nickel-titanium alloy (UNS N01555). The engineers have equipped an artificial hand with ‘muscles’ that are composed of ultrafine nickel-titanium shape-memory alloy (SMA) wires with a diameter similar to that of a human hair. Like a muscle, the wires can contract and relax.

A tool fabricated with artificial muscles from SMA wire would be light, flexible and highly adaptable. And these wires “have the highest energy density of all known drive mechanisms, which enables them to perform powerful movements in restricted spaces,” explains Seelecke. The term ‘shape memory’ refers to the fact that the wire is able to ‘remember’ and return to its original predetermined shape after it has been deformed. “This property of nickel-titanium alloy is a result of phase changes that occur within the material. If the wire becomes warm, which happens, for instance, when it conducts electricity, the material transforms its lattice structure causing it to contract like a muscle,” says Seelecke.

The engineers copied the structure of natural human muscles by grouping the very fine yet strong wires into bundles to mimic muscle fibres. The bundle can contract and relax quickly while exerting a high tensile force, because rapid cooling is possible as lots of individual wires present a greater surface area through which heat can be dissipated. Unlike a single thick wire, a bundle of very fine wires can undergo rapid contractions and extensions equivalent to those observed in human muscles. As a result, the researchers were able to achieve fast and smooth finger movements.

The researchers want to continue developing the prototype and improve the way in which it simulates the human hand. This will involve modelling hand movement patterns and exploiting the sensor properties of nickel-titanium SMA wire. This nickel-containing technology could eventually be used to build prosthetic limbs with a more natural function and feel.

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